Radiocarbon Dating

Radiocarbon dating is one of the most widely used scientific dating methods in archaeology and environmental science. It can be applied to most organic materials and spans dates from a few hundred years ago right back to about 50,000 years ago - about when modern humans were first entering Europe.

For radiocarbon dating to be possible, the material must once have been part of a living organism. This means that things like stone, metal and pottery cannot usually be directly dated by this means unless there is some organic material embedded or left as a residue.

As explained below, the radiocarbon date tells us when the organism was alive (not when the material was used). This fact should always be remembered when using radiocarbon dates. The dating process is always designed to try to extract the carbon from a sample which is most representative of the original organism.

In general it is always better to date a properly identified single entity (such as a cereal grain or an identified bone) rather than a mixture of unidentified organic remains.

The radiocarbon formed in the upper atmosphere is mostly in the form of carbon dioxide. This is taken up by plants through photosynthesis. Because the carbon present in a plant comes from the atmosphere in this way, the radio of radiocarbon to stable carbon in the plant is virtually the same as that in the atmosphere.

Plant eating animals (herbivores and omnivores) get their carbon by eating plants. All animals in the food chain, including carnivores, get their carbon indirectly from plant material, even if it is by eating animals which themselves eat plants. The net effect of this is that all living organisms have the same radiocarbon to stable carbon ratio as the atmosphere.

Once an organism dies the carbon is no longer replaced. Because the radiocarbon is radioactive, it will slowly decay away. Obviously there will usually be a loss of stable carbon too but the proportion of radiocarbon to stable carbon will reduce according to the exponential decay law:

R = A exp(-T/8033)

where R is 14C/12C ratio in the sample, A is the original 14C/12C ratio of the living organism and T is the amount of time that has passed since the death of the organism.

By measuring the ratio, R, in a sample we can then calculate the age of the sample:

The simplified approach described above does not tell the whole story. There are two reasons why the radiocarbon date is not a true calendar age:

Half life:
this is not exactly as originally measured by Libby; the original half life is still used in calculations in order to maintain consistency and because other effects are more important

Atmospheric variations:
the radiocarbon concentration of the atmosphere has not always been constant; in fact it has varied significantly in the past

Both of these complications are dealt with by calibration of the radiocarbon dates against material of know age.

Further complications arise when the carbon in a sample has not taken a straightforward route from the atmosphere to the organism and thence to the measured sample. Common examples are:

Contamination:
where material from the soil or conservation work becomes incorporated into the sample resulting in an admixture of carbon with a different radiocarbon content; the purpose of chemical pre-treatment is to remove all such material

Reservoir effects:
these occur, for example, when some of the carbon reaches the sample by way of the oceans; because the radiocarbon composition of the oceans differs from that of the atmosphere, this can lead to erroneous dates; stable isotope measurements can be used to see if this effect is present since the stable isotope concentration of the oceans is also different